Covert surveillance using multi-modality sensing

ABSTRACT

The present specification discloses a covert mobile inspection vehicle with a backscatter X-ray scanning system that has an X-ray source and detectors for obtaining a radiographic image of an object outside the vehicle. The systems preferably include at least one sensor for determining a distance from at least one of the detectors to points on the surface of the object being scanned, a processor for processing the obtained radiographic image by using the determined distance of the object to obtain an atomic number of each material contained in the object, and one or more sensors to obtain surveillance data from a predefined area surrounding the vehicle.

CROSS-REFERENCE

The present specification is a continuation of U.S. patent applicationSer. No. 13/368,202, of the same title, and filed on Feb. 7, 2012,which, in turn, relies on U.S. Provisional Patent Application No.61/440,834, filed on Feb. 8, 2011, and entitled “Covert SurveillanceUsing Multi-Modality Sensing”, for priority. The aforementionedapplication is incorporated herein by reference in its entirety.

FIELD

The present specification generally relates to the field of covertsurveillance for detecting threat items and contraband, either in avehicle or on a person, and more specifically to a covert mobileinspection vehicle which combines a plurality of detection andprevention components that may be deployed rapidly to a threat zone toaid detection and prevention of subversive activities.

BACKGROUND

Currently, there exists the threat of terrorism. To counter this threat,there is a requirement for systems to be put in place to detect and dealwith subversive activity. Some of such systems known in the art arepurely designed to detect subversive activity; others are designed toprevent subversive activity; while still other known systems aredesigned purely as a deterrent. For example, some systems are primarilyphysical (such as barriers and security agents), some rely on networksof sensors (such as CCTV systems) while others involve dedicatedinstallations (such as radio jamming mast or X-ray scanning machines).

What is needed, however, are covert surveillance systems that can beeffectuated with high mobility and speedy deployment and that allow theuse of a plurality of surveillance data to enable more informed, robustand intelligent threat detection and prevention.

Accordingly, there is need for a covert mobile inspection vehicle thatuses a plurality of prevention and detection components or sensors.

There is also need for a system that intelligently integrates and/orcorrelates surveillance information from the plurality of multi-modalitysensors to detect and prevent subversive activities.

SUMMARY

In one embodiment, the present specification discloses a covert mobileinspection vehicle comprising: a backscatter X-ray scanning systemcomprising an X-ray source and a plurality of detectors for obtaining aradiographic image of an object outside the vehicle; at least one sensorfor determining a distance from at least one of the plurality ofdetectors to points on the surface of the object; a processor forprocessing the obtained radiographic image by using the determineddistance of the object to obtain an atomic number of each materialcontained in the object; and one or more sensors to obtain surveillancedata from a predefined area surrounding the vehicle. In an embodiment,the sensor is a scanning laser range finder causing a beam of infra-redlight to be scattered from the surface of the object wherein a timetaken for the beam of infra-red light to return to the sensor isindicative of the distance to the surface of the object.

Further, in an embodiment, the processor causes an intensity correctionto be applied to the obtained radiographic image thereby causingintensity of the image of an object located at a distance greater than apredefined distance to be reduced by a predefined factor and intensityof the image of an object located at a distance lesser than a predefineddistance to be increased by a predefined factor.

In another embodiment, the processor implements an adaptive region basedaveraging method whereby backscattered X-rays from a first region of theobject located at a distance greater than a predefined distance from theX-ray source are averaged over a second larger predefined region,causing a linear dimension of the first region to be scaled as a squareof the distance from X-ray source to the object. In yet anotherembodiment, the adaptive region averaging method is implemented by usinga statistical filter to determine if a first pixel of an obtainedradiographic image is part of a first object or of a second adjacentobject. Also, in an embodiment, one or more individual pixels in theobtained radiographic image are colored based on the determined distanceof the object and an atomic number of each material contained in theobject.

In another embodiment, the obtained distance of the object is used toprovide a geometric correction to produce a true likeness of the shapeof the object. In yet another embodiment, the X-ray source comprises anX-ray tube having a cathode-anode potential difference ranging from 160kV to 320 kV and a tube current ranging from 1 mA to 50 mA for producinga broad spectrum of X-ray energies.

Further, in an embodiment, the plurality of detectors comprise one of:an inorganic scintillation detector such as NaI(Tl), and an organicscintillator such as polyvinyl toluene; each detector being coupled withone or more light sensitive readout devices such as a photomultipliertube or a photodiode. Also, in an embodiment, the plurality of detectorscomprise semiconductor sensors having a wide bandgap such as, CdTe,CdZnTe or HgI which can operate at room temperature. In anotherembodiment, the plurality of detectors comprises semiconductor sensorshaving a narrow bandgap such as HPGe.

In yet another embodiment, the covert mobile inspection vehicle furthercomprises a data acquisition module comprising a plurality of detectors,photomultipliers/photodiodes and analog-to-digital converter circuitry.Also in an embodiment, the covert mobile inspection vehicle comprises atleast one of: a GPS receiver, a scanning laser, a CCTV camera, aninfra-red camera, an audio microphone, a directional RF antenna, awide-band antenna, a chemical sensor, and a jamming device. In anotherembodiment, the covert mobile inspection vehicle also comprises anautomated detection processor for: integrating and analysing in realtime the surveillance data from the one or more sensors; and sendingthreat items obtained by analyzing the surveillance data for review toan operator via wired or wireless means. In another embodiment, thecovert mobile inspection vehicle further comprises means forbroadcasting the surveillance data to a central intelligence location inreal time.

In an embodiment, the X-ray source comprises a multi-element scattercollimator to produce a fan beam of X rays for irradiating the objectbeing scanned; backscattered X rays from the object being detected by asegmented detector array located behind the multi-element collimator andcomprising one detector element corresponding to each collimatorelement.

In an embodiment, the present specification describes a method forobtaining an atomic number (Z) of each material contained in an objectbeing scanned by a covert mobile inspection vehicle comprising: abackscatter X-ray scanning system comprising an X-ray source and aplurality of detectors for obtaining a radiographic image of the object;at least one sensor for determining a distance from at least one of theplurality of detectors to points on the surface of the object; and aprocessor for processing the obtained radiographic image by using thedetermined distance of the object to obtain an atomic number of eachmaterial contained in the object; the method comprising the steps of:determining a true extent of each region of the radiographic image byusing a statistical filter; calculating a standard deviation of energiesof pixels present in each region; calculating a product of the obtainedstandard deviation and a mean of the energies of pixels present in eachregion; and comparing the calculated product to a pre-determined scalewhere a low value of the product corresponds to a low Z material and ahigh value of the product corresponds to a high Z material.

In another embodiment, the present invention discloses a covert mobileinspection vehicle comprising: a backscatter X-ray scanning systemcomprising an X-ray source and a plurality of detectors for obtaining aradiographic image of an object outside the vehicle; and one or moresensors to obtain surveillance data from a predefined area surroundingthe vehicle. In an embodiment, at least one of the sensors is a scanninglaser range finder causing a beam of infra-red light to be scatteredfrom the surface of the object; a time taken for the beam of infra-redlight to return to the sensor is indicative of the distance to thesurface of the object.

In an embodiment, the vehicle further comprises at least one sensor fordetermining a distance from at least one of the plurality of detectorsto points on the surface of the object; and a processor for processingthe obtained radiographic image by using the determined distance of theobject to obtain an atomic number of each material contained in theobject. In yet another embodiment, the processor causes an intensitycorrection to be applied to the obtained radiographic image therebycausing intensity of the image of an object located at a distancegreater than a predefined distance to be reduced by a predefined factorand intensity of the image of an object located at a distance lesserthan a predefined distance to be increased by a predefined factor.

In another embodiment, the plurality of detectors comprise one of: aninorganic scintillation detector such as NaI(Tl), and an organicscintillator such as polyvinyl toluene; each detector being coupled withone or more light sensitive readout devices such as a photomultipliertube or a photodiode. In yet another embodiment, the plurality ofdetectors comprise semiconductor sensors having a wide bandgap such as,CdTe, CdZnTe or HgI which can operate at room temperature. In a furtherembodiment, the plurality of detectors comprises semiconductor sensorshaving a narrow bandgap such as HPGe.

In an embodiment, the covert mobile inspection vehicle further comprisesa data acquisition module comprising a plurality of detectors,photomultipliers/photodiodes and analog-to-digital converter circuitry.In another embodiment, the covert mobile inspection vehicle furthercomprises at least one of: a GPS receiver, a scanning laser, a CCTVcamera, an infra-red camera, an audio microphone, a directional RFantenna, a wide-band antenna, a chemical sensor, and a jamming device.

The aforementioned and other embodiments of the present shall bedescribed in greater depth in the drawings and detailed descriptionprovided below.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will befurther appreciated, as they become better understood by reference tothe detailed description when considered in connection with theaccompanying drawings:

FIG. 1A is an illustration of a covert mobile inspection vehicle, inaccordance with an embodiment of the present invention;

FIG. 1B is a schematic representation of one embodiment of a four-sidedX-ray imaging system that may be employed in accordance with the presentinvention;

FIG. 1C is an illustration of an embodiment of the X-ray scanning systemon-board the surveillance vehicle of FIG. 1A in accordance with oneembodiment of the present invention;

FIG. 2A depicts a representation, as a step function, of an X-ray sourcebeing switched rapidly from its beam-off condition to its beam-oncondition, that may be employed in accordance with the presentinvention;

FIG. 2B diagrammatically illustrates an operation of time of flightbackscatter imaging, that may be employed in accordance with the presentinvention;

FIG. 3A depicts a backscatter radiographic image without using intensityor effective atomic number scaling;

FIG. 3B depicts a backscatter radiographic image where intensity ofobject images has been scaled for distance, in accordance with anembodiment of the present invention;

FIG. 3C depicts a backscatter radiographic quantitative image scaled byeffective atomic number, in accordance with an embodiment of the presentinvention;

FIG. 4 is a graphical representation of a Bremsstrahlung spectrum with atypical tungsten anode X-ray tube;

FIG. 5 is a graphical representation of a high mean energy spectrum forhigh Z materials and a low mean energy spectrum for lower Z materials,in accordance with an embodiment of the present invention;

FIG. 6 is a graphical representation of a gamma ray spectrum with higherenergies as compared with X-rays, in accordance with an embodiment ofthe present invention;

FIG. 7 is a flowchart illustrating a method of obtaining an atomicnumber of each material contained in an object being scanned by thecovert mobile inspection vehicle of the present invention;

FIG. 8 is an illustration of another embodiment of the covert mobileinspection vehicle, shown in FIG. 1, further illustrating an on-boardX-ray scanning system;

FIG. 9 is a schematic representation of components of a scanning systemthat may be employed in accordance with the present invention;

FIG. 10 is a schematic representation of components of a scanning systemthat may be employed in accordance with the present invention;

FIG. 11 is a schematic representation of components of a scanning systemthat may be employed in accordance with the present invention;

FIG. 12 shows a schematic view of a detector element that may beemployed in accordance with the present invention; and

FIG. 13 is a schematic representation of a radiation imaging system thatmay be employed in accordance with the present invention.

DETAILED DESCRIPTION

The present specification is directed towards a covert mobile inspectionsystem, comprising a vehicle, which is equipped with a plurality ofmulti-modality sensors. Surveillance information from the plurality ofsensors is utilized to detect and prevent subversive activities. Thus,the present specification describes a system and method for providingcovert and mobile surveillance/inspection of subversive activities usinga plurality of multi-modality surveillance sensors.

In addition, the present specification is directed toward using abackscatter X-ray scanning system that has improved threat detectioncapabilities as at least one of the plurality of surveillance sensorsutilized.

Accordingly, in one embodiment, the present specification describes acovert mobile inspection vehicle having an improved on-board backscatterX-ray scanning system and further equipped with a plurality ofprevention and inspection components or devices.

In one embodiment, the backscatter X-ray scanning system includes asensor, such as a scanning laser range finder, that measures thedistance of the detectors from the surface of the object underinspection.

Because it is possible to map the equivalent distance between the X-raybeam at any angle and the surface of the object by determining therelative positions of the X-ray source and the laser sensor, in oneembodiment, the present specification describes an improved method ofgenerating a radiographic image of the object under inspection, usingthis known distance to generate an intensity-corrected image at a givenequivalent distance. The corrected image is then used to map aneffective atomic number of all materials in the radiographic image.Additionally, this distance data is also used to provide an accurategeometric correction in the image to produce a true likeness of theshape of the object under inspection.

In another aspect of the improved method of generating a radiographicimage of the object under inspection, adaptive region based averaging isapplied (such as by using a statistical filter and/or median filter).This results in an image which has equivalent statistical propertiesuseful in determining an accurate effective atomic number for allregions in the object under investigation. Optionally, the knowledge ofeffective atomic numbers and their ranges or variations is used tocolour code the radiographic image.

In another embodiment, the present specification describes a method formeasuring individual X-ray energies as they interact within at least onedetector in order to form an analysis of the spectral content of thescattered X-ray beam.

In another embodiment, the backscatter X-ray scanning systemadditionally uses a multi-element scatter collimator to allow use offan-beam X-ray irradiation to generate the backscatter image. Therefore,scattered X-rays which lie within an acceptance angle of, for example,the collimator element are detected and associated to the appropriatecorresponding part of the generated radiographic X-ray image.

Apart from the X-ray scanner/sensor, the plurality of multi-modalitysurveillance sensors comprise any or all combinations of components suchas GPS receivers, scanning lasers, CCTV cameras, infra-red cameras,audio microphones, directional RF antennas, wide-band antennas, chemicalsensors, jamming devices.

In accordance with another embodiment, the present specificationdescribes an automated detection processor for integrating and analysingall surveillance information from the plurality of sensors, inreal-time, to highlight threat items for review by an operator seatedinside the covert vehicle and/or remotely through a secured wirelessnetwork.

The present specification discloses multiple embodiments. The followingdisclosure is provided in order to enable a person having ordinary skillin the art to practice the invention. Language used in thisspecification should not be interpreted as a general disavowal of anyone specific embodiment or used to limit the claims beyond the meaningof the terms used therein. The general principles defined herein may beapplied to other embodiments and applications without departing from thespirit and scope of the invention. Also, the terminology and phraseologyused is for the purpose of describing exemplary embodiments and shouldnot be considered limiting. Thus, the present invention is to beaccorded the widest scope encompassing numerous alternatives,modifications and equivalents consistent with the principles andfeatures disclosed. For purpose of clarity, details relating totechnical material that is known in the technical fields related to theinvention have not been described in detail so as not to unnecessarilyobscure the present invention.

FIG. 1A shows a covert mobile inspection system 100 in accordance withan embodiment of the present invention. The system 100 comprises arelatively small vehicle 102, such as a van, which is equipped with aplurality of detection and prevention sensors 104 such as scanning,listening and broadcasting devices. In an embodiment, the vehicle is a3.5 ton chassis having a height less then 3 m above road level, lengthranging from 4 m to 6 m and width ranging from 2.2 m to 2.5 m. In otherembodiments, the vehicle may comprise small vans having a weight rangingfrom 1.5 T to 3.5 T. One aspect of the embodiments disclosed herein isthe use of surveillance data from these multi-modality sensors incorrelation and/or aggregation with data from an on-board X-ray scanningsensor. In one embodiment of the present invention, the X-ray scanningsystem on-board the surveillance vehicle of FIG. 1A also comprises asensor in order to measure its distance to the scattering object,material or point.

In one embodiment, the X-ray sensor generates a backscatter radiographicimage of an object from a single side utilizing Compton scattering. Thisallows the vehicle 105 to collect scan data, in a covert fashion, at alow dose to allow scanning of individuals, small as well as largevehicles/cargo for detection of threat devices, materials andindividuals.

In another embodiment, the X-ray scanning system allows for scanning ofseveral sides of a vehicle under inspection. For example, U.S. patentapplication Ser. No. 12/834,890 and Patent Cooperation Treaty (PCT)Application Number US10/41757 both entitled “Four-Sided Imaging”, andfiled on Jul. 12, 2010 by the Applicant of the present specification,both herein incorporated by reference in their entirety, describe “[a]scanning system for the inspection of cargo, comprising: a portaldefining an inspection area, said portal comprising a first verticalside, a second vertical side, a top horizontal side, and a horizontalbase defined by a ramp adapted to be driven over by a vehicle; a firstX-ray source disposed on at least one of the first vertical side, secondvertical side or top horizontal side for generating an X-ray beam intothe inspection area toward the vehicle; a first set of transmissiondetectors disposed within the portal for receiving the X-raystransmitted through the vehicle; a second X-ray source disposed withinthe ramp of said portal for generating an X-ray beam towards theunderside of the vehicle; and a second set of detectors disposed withinthe ramp of said portal for receiving X-rays that are backscattered fromthe vehicle.

FIG. 1B is a schematic representation of one embodiment of thefour-sided X-ray imaging system 100B disclosed in U.S. patentapplication Ser. No. 12/834,890 and Patent Cooperation Treaty (PCT)Application Number US10/41757. As shown in FIG. 1B, vehicle 105 drivesover a ramp 110 and underneath an archway 115, which defines aninspection portal. Specifically, the portal is defined by a first (left)side, a second (right) side, a top side and a bottom platform, which isa portion of the ramp 110. In one embodiment, ramp 110 comprises a base,a first angled surface leading upward to a flat transition pointdefining the highest part of the ramp, which also functions as thebottom platform, and a second angled surface leading back down to theground. The highest part of the ramp is typically between 50 and 150 mmin height. In one embodiment, archway 115 houses multiple X-raytransmission detectors 117 and at least one X-ray source 119, housedwithin an enclosure, shown as 220 in FIG. 2.

While FIG. 1B depicts the X-ray source 119 as being on the left side ofthe portal, one of ordinary skill in the art would appreciate that itcould be on the right side, with an appropriate reconfiguration of thedetectors 117. Preferably, the enclosure housing the X-ray is physicallyattached to the exterior face of the first side and is approximately 1meter tall. The position of the enclosure depends upon the size of theinspection portal. In one embodiment, the enclosure occupies 20% to 50%of the total height of the first side. In one embodiment, a slit oropening is provided on first side, through which X-rays are emitted.Slit or opening extends substantially up first side to approximately100% of the height. In one embodiment, slit or opening is covered with athin coating that is substantially transparent to an X-ray. In oneembodiment, the thin coating is comprises of a material such asaluminium or plastic and further provides an environmental shield.

In one embodiment, the enclosure and X-ray unit further comprise a firstcollimator close to the source of X-rays and a second collimator closeto the exit, described in greater detail below. Where the X-ray sourceenclosure is so positioned, detectors 117 are positioned on the interiorface of the second side and the interior face of top side and occupy thefull height of second side and the full length of top side, proximate tosecond side.

In another embodiment, the enclosure housing the X-ray is physicallyattached to the exterior face of the second side and is approximately 1meter tall. The position of the enclosure depends upon the size of theinspection portal. In one embodiment, the enclosure occupies 20% to 50%of the total height of the first side. As described above with respectto first side, if the enclosure housing the X-ray is on second side, aslit or opening is similarly provided on second side. The detectors arealso similarly positioned on the interior faces of top side and firstside when the enclosure is on second side. In one embodiment, with adual-view system, an enclosure housing an X-ray source can be providedon both the first side and second side.

As shown in FIG. 1C, the X-ray scanning system 200 comprises an X-raysource 205 collimated by a rotating disk with a small aperture whichallows X-rays to scan in at least one pencil beam 206, and preferably aseries of “moving” pencil beams, within a substantially vertical planefrom the X-ray source 205 to the object 210. X-rays 207 scatter backfrom the object 210 under inspection and some of these reach at leastone detector array 215 located adjacent to the X-ray source 205 butoutside the plane described by the moving X-ray beam 206. The intensityof the backscatter signal 207 is representative of the product ofdistance to the object and atomic number of the object.

Persons of ordinary skill in the art would appreciate that the signalsize due to Compton scattering from objects varies as the inverse fourthpower of distance between the X-ray source and the scattering object. Itis also known to persons of ordinary skill in the art that low atomicnumber materials are less efficient at scattering X-rays than highatomic number materials while high atomic number materials are moreefficient at absorbing X-rays of a given energy than low atomic numbermaterials. Therefore, the net result is that more X-rays having agreater intensity are scattered from low atomic number materials thanfrom high atomic number materials. However, this effect variesapproximately linearly with atomic number while the X-ray signal variesas the inverse fourth power of distance from the source to thescattering object. This also implies that known Compton scatter basedradiographic images are essentially binary in nature (scattering or notscattering) since the small but quantitative variation of the signalsize due to variation in atomic number is lost in the gross variation insignal intensity caused due to varying distances from X-ray source toscattering points.

To correct for distance, a sensor 220 is provided (adjacent to the X-raysource and detectors) which is capable of detecting the distance to eachpoint at the surface of the object 210. In one embodiment, the sensor220 is advantageously a scanning laser range finder in which a beam ofinfra-red light 221 is scattered from the surface of the object 210 andthe time taken for the pulsed beam to return to the sensor 220 isindicative of the distance to the surface of the object 210. Forexample, U.S. patent application Ser. No. 12/959,356 and PatentCooperation Treaty Application Number US10/58809, also by the Applicantof the present specification, entitled “Time of Flight BackscatterImaging System” and filed on Dec. 22, 2010, both of which are hereinincorporated by reference in their entirety, describes a method in whichthe time of flight of the X-ray beam to and from the surface of theobject under inspection is used to determine the distance between thesource and scattering object.

One of ordinary skill in the art would note that the distances betweenthe surface of the object and the planar detector arrays are variable,since the object is not straight sided. Further, since the distance fromthe X-ray source to the object under inspection is not known in general,an assumption is generally made that the object is planar and at a fixeddistance from the source. Thus, if the object is closer than assumed,then the object will appear smaller in the image and conversely, if theobject is further away then it will appear to be larger. The result isan image which is representative of the object under inspection but notwith correct geometry. This makes it difficult to identify the preciselocation of a threat or illicit object within the object underinspection.

U.S. patent application Ser. No. 12/959,356 and Patent CooperationTreaty Application Number US10/58809 address the above problem byintegrating time of flight processing into conventional backscatterimaging. X-rays travel at a constant speed which is equal to the speedof light (3×10⁸ m/s). An X-ray will therefore travel a distance of 1 min 3.3 ns or equivalently, in 1 ns (10⁻⁹ s) an X-ray will travel 0.3 m.Thus, if the distance between a backscatter source and the object underinspection is on the order of 1 m, it corresponds to around 3 ns oftransit time. Similarly, if the backscatter X-ray detector is alsolocated around 1 m from the surface of the object, it corresponds to anadditional 3 ns of transit time. Thus, the signal received at thedetector should be received, in this example, 6 ns after the X-ray beamstarted its transit from the X-ray tube. In sum, the X-ray's transittime is directly related to the detectors' distance to or from theobject. Such times, although quite short, can be measured usingdetection circuits known to those of ordinary skill in the art.

The minimum distance is practically associated with the time resolutionof the system. Objects can be proximate to the source, but one will notsee much scattered signal since the scatter will generally be directedback to the X-ray source rather than to a detector. A practical lowerlimit, or the minimum distance between the plane of the system and thenearest part of the object to be inspected, is 100 mm. The further awaythe object is from the detector, the smaller the signal size and thus apractical upper limit for distance is of the order of 5 m.

In the systems of the present application, as shown diagrammatically inFIGS. 2A and 2B, the distance between the X-ray source and the objectunder inspection is determined precisely by recording the time taken foran X-ray to leave the source and reach the detector. FIG. 2A depicts arepresentation, as a step function, of an X-ray source being switchedrapidly from its beam-off condition to its beam-on condition. While 201represents the step function at the source, 202 represents thedetector's response. Thus, as can be seen from 201 and 202, after thebeam is switched on from its off state at the source, the detectorresponds with a step-function like response after a time delay Δt 203.Referring to FIG. 2B, as the source 209 emits a pencil beam 211 ofX-rays towards the object 212, some of the X-rays 213 transmit into theobject 212, while some X-rays 214 backscatter towards the detectors 217.

It may be noted that there are different path lengths from the X-rayinteraction point (with the object) to the X-ray detector array.Therefore if a large detector is used, there will be a blurring to thestart of the step pulse at the detector, where the leading edge of thestart of the pulse will be due to signal from the part of the detectorwhich is nearest to the interaction spot, and the trailing edge of thestart of the pulse will be due to signal from parts of the detectorwhich are further away from the interaction spot. A practical system canmitigate such temporal blurring effects by segmenting the detector suchthat each detector sees only a small blurring and the changes inresponse time each provide further enhancement in localisation of theprecise interaction position, hence improving the determination of thesurface profile of the object under inspection.

The detector size (minimum and/or maximum) that would avoid suchblurring effects described above is commensurate with the timeresolution of the system. Thus, a system with 0.1 ns time resolution hasdetectors of the order of 50 mm in size. A system with 1 ns timeresolution has detectors of the order of 500 mm in size. Of course,smaller detectors can be used to improve statistical accuracy in thetime measurement, but at the expense of reduced numbers of X-ray photonsin the intensity signal, so there is a trade-off in a practical systemdesign which is generally constrained by the product of sourcebrightness and scanning collimator diameter.

Referring to FIG. 1C, it should be appreciated that knowing the relativepositions of the X-ray source 205 and the laser sensor 220 theequivalent distance between the X-ray beam 206 at any angle and thesurface of the object 210 is mapped using a geometric look up table (forcomputational efficiency). This known distance is then used to apply anintensity correction to the measured X-ray scatter data to produce aradiographic image at a given equivalent distance of, say, 1 m. Thus,objects that are closer than 1 m will have their intensity reduced by afactor of 1/(1-distance)⁴ while objects farther away than 1 m will havetheir intensity increased by a factor of 1/(1-distance)⁴. Thequantitatively corrected image so produced is then used to map aneffective atomic number of all materials in the radiographic image, asshown in FIGS. 3A through 3C.

As shown in FIG. 3A, radiographic image 305 represents an image of twoobjects obtained using an X-ray scanning system without intensity oreffective atomic number scaling, the lower one 302 being close to theX-ray source and the upper one 304 being farther away from the source.The lower object 302 is shown to be bright while the upper image 304 isseen to be faint.

Referring now to FIG. 3B, image 310 shows the result of scalingintensity for distance where the lower object 307 is now lighter than inimage 305 while the upper object 308 is now brighter than the lowerobject 307. This suggests that the upper object 308 is of lower atomicnumber than the lower object 307. This is in contrast to the originalimage 305, wherein the relative atomic numbers are typically prone tomisrepresentation.

In accordance with another aspect of the present application, it isrecognized that signal scattered due to objects farther from the X-raysource have poorer signal-to-noise ratio than signal from scatteringobjects closer to the source. This implies that the distance measurementcan be further utilized to implement an adaptive region based averagingmethod whereby signal from regions far from the source are averaged overa larger region, such that the linear dimension of these regions isscaled as the square of the distance from source to object. This effectis shown in image 315 of FIG. 3C. In FIG. 3C, the upper object 313 hasbeen averaged over larger regions than the lower object 312 therebyresulting in equivalent statistical properties useful in determining anaccurate effective atomic number for all regions in the object underinvestigation. In a preferred embodiment, the adaptive region averagingmethod is implemented using a statistical filter to determine if a givenpixel is likely to be a part of the main scattering object, or part ofan adjacent object in which this value should not be used to compute theregion average.

In one embodiment, a suitable statistical filter lists all pixel valueswithin a region (for example a 7×7 block), ranks them in order and thendetermines the mean value and standard deviation of the central range ofvalues. Any pixel within the whole block whose intensity is more than 2standard deviations from the mean value within that block is consideredto be part of an adjacent object. A range of statistical filters can bedeveloped which may use higher order statistical attributes, such asskewness, to refine the analysis. Alternate methods, such as medianfiltering, which can mitigate against boundary effects between imagefeatures are well known to persons of ordinary skill and all suchmethods can be suitably applied within the scope of the presentinvention.

In accordance with yet another aspect described in the presentspecification, in one embodiment, the individual pixels in image 310 arecolored according to the values in the quantitative image 315 scaled byeffective atomic number. Here, the distance normalized pixels arecolored on an individual basis (to ensure a sharp looking image) basedon results from the region averaged image 315 with improved statistics.Alternative schemes can also be used for pixel coloring. For example,pixels with effective atomic number below 10 are colored orange(corresponding to organic materials such as explosives), pixels witheffective atomic numbers between 10 and 20 are colored green(corresponding to low atomic number inorganic materials such asnarcotics) while materials with effective atomic numbers greater than20, such as steel, are colored blue. Still alternatively, a rainbowspectrum can be used in which pixel colored changes from red throughyellow, green and blue as effective atomic number increases. Many othercolor tables can be selected depending on preference and application.

In accordance with further aspect of the present specification, it isrecognized that the beam from the X-ray source is diverging from a pointwhich is generally located at least one meter from ground level. Thisimplies that the raw image 305 is actually distorted—with regions at thecentre of the image being unnaturally wide compared to regions at thetop and bottom of the image which are unnaturally narrow. Inconventional methods, a geometric correction is applied according to acosine-like function which makes the assumption of a flat sided objectat a fixed distance from the source. In contrast, in an embodiment ofthe present invention, the distance data from the scanning laser sensor220 of FIG. 1C is used to provide an accurate geometric correction toproduce a true likeness of the shape of the object under inspection.

The present invention also lays focus on spectral composition of theX-ray beam that is incident on the object under inspection. Accordingly,in one embodiment it is advantageous to create the X-ray beam using anX-ray tube with cathode-anode potential difference in the range 160 kVto 320 kV with tube current in the range of 1 mA to 50 mA depending onallowable dose to the object under inspection and weight and powerbudget for the final system configuration. Regardless of tube voltageand current, a broad spectrum of X-ray energies is produced as shown inFIG. 4. Here, a broad Bremsstrahlung spectrum 405 is visiblecomplimented by fluorescence peaks 410 at 60 keV with a typical tungstenanode tube.

It should be noted that as a result of Compton scattering, the X-raysbackscattered towards the detectors are generally of lower energy thanthose interacting in the object itself, and so the scattered beam has alower mean energy than the incident beam. Further, the impact of thescattering object is to preferentially filter the X-ray beam—removingmore and more of the lower energy components of the beam the higher theeffective atomic number of the scattering object. This phenomenon isshown in FIG. 5 where a high atomic number (Z) material representshigher mean energy spectrum 505 while a lower atomic number (Z) materialis represented by the relatively lower mean energy spectrum 510, therebyenabling discerning of low Z items from relatively high Z items.

Referring back to FIG. 1C, the detectors 215 measure the energy of theX-rays 207 that arrive at the detectors 215 after being scattered by theobject 210. In one embodiment, each detector 215 comprises an inorganicscintillation detector such as NaI(Tl) or an organic scintillator suchas polyvinyl toluene coupled directly to one or more light sensitivereadout devices such as a photomultiplier tube or a photodiode. In analternate embodiment, the detectors comprise semiconductor sensors suchas semiconductors having a wide bandgap including, but not limited to,CdTe, CdZnTe or HgI which can operate at room temperature; orsemiconductors having a narrow bandgap such as, but not limited to, HPGewhich needs to be operated at low temperatures. Regardless of thedetector configuration chosen, the objective is to measure individualX-ray energies as they interact in the detector in order to form ananalysis of the spectral content of the scattered X-ray beam 207.

Persons of ordinary skill in the art would appreciate that the dataacquisition module (typically comprising detectors,photomultipliers/photodiodes and analog-to-digital converter circuitryand well known to persons skilled in the art) will be synchronized tothe position of the primary X-ray beam 206 in order to collect onespectrum for each interacting X-ray source point. For example, the X-raysystem 200 may be configured to collect 300 lines per second with 600pixels per image line. In this case, the equivalent dwell time of theprimary X-ray beam at each source point is 1/180000 sec=5.5 μs per pointand the detectors need to be capable of recording several hundred X-raysduring this time. To achieve the necessary count rates, one embodimentuses a small number of fast responding detectors (such as polyvinyltoluene plastic scintillators with photomultiplier readout) or a largernumber of slow responding detectors (such as NaI scintillators withphotomultiplier readout), depending upon factors such as cost andcomplexity.

Given the acquisition of the X-ray spectrum at each sample point and thephenomena described with reference to FIGS. 4 and 5, it would be evidentto those of ordinary skill in the art that the statistical properties ofthe X-ray spectrum can provide additional information on the effectiveatomic number of the scattering material at each primary beaminteraction site. Using the known distance information, the area of thespectrum may be corrected to yield an improved quantitative result (asdiscussed earlier), while properties such as mean energy, peak energyand skewness of the spectrum provide the quantitative parameters thatare required for accurate materials analysis.

As an example, a scattering object far from the detector will produce anaturally faint signal, with the displayed brightness of this objectbeing corrected through the use of known distance information, such asthat provided by a scanning laser. Given that the signal for the regionis formed from a limited number of scattered X-ray photons, theproperties of the signal can be described using Gaussian statistics.Gain correction to account for distance from the source is applied in alinear fashion, and so the region still maintains its originalstatistical properties even though its mean value has been scaled to alarger value.

As identified in FIG. 5, the spectral composition of the scattered beamis dependent on effective atomic number of the scattering material. FIG.7 is a flowchart illustrating a method of obtaining an atomic number ofeach material contained in an object being scanned by the covert mobileinspection vehicle of the present invention. At step 702, a true extentof each region of the radiographic image is obtained by using a suitablestatistical filter as described earlier. A true extent of a regionenables determining a boundary of each constituent material. Thus, thetrue extent refers to the physical area over which the object extends.It is desirable to find the point at which one object finishes and atwhich the next object begins so that only pixels for the current objectare used in quantitative imaging, without the effects of contaminationfrom adjacent objects. At step 704, a mean energy of each detectedsignal is calculated along with a standard deviation and skewness ofenergies of pixels present in each region. At step 706, a product of thecalculated standard deviation and a mean energy of the pixels energiesof pixels present in each region is calculated. At step 708, thecalculated product is compared with a pre-determined scale where a lowvalue of the product corresponds to a low atomic number material and ahigh value of the product corresponds to a high atomic number material.

Further, it should be appreciated that the X-ray scatter data isgenerally at low energy and often below 100 keV in magnitude. Incontrast, gamma-rays from radioactive sources, that may be present inthe object under inspection, will typically be at much higher energy(for example Co-60 has gamma-rays at 1.1 and 1.3 MeV while Cs-137 emitsgamma rays at 662 keV). As shown in FIG. 6, it is therefore possible todiscriminate these high energy gamma rays, represented by spectrums 605and 606, from the low energy scattered X-rays 610 thereby allowingsimultaneous acquisition of active X-ray backscatter signals along withpassive gamma-ray detection in accordance with an aspect of the presentinvention.

U.S. patent application Ser. No. 12/976,861, also by the Applicant ofthe present invention, entitled “Composite Gamma Neutron DetectionSystem” and filed on Dec. 22, 2010, describes a method for simultaneousdetection of gamma-rays and neutrons with pulse shape discrimination todiscriminate between the two effects. This method is also applicable tothe current invention and is incorporated herein by reference.

As described in U.S. patent application Ser. No. 12/976,861, severalnuclei have a high cross-section for detection of thermal neutrons.These nuclei include He, Gd, Cd and two particularly high cross-sectionnuclei: Li-6 and B-10. In each case, after the interaction of a highcross-section nucleus with a thermal neutron, the result is an energeticion and a secondary energetic charged particle.

For example, the interaction of a neutron with a B-10 nucleus can becharacterized by the following equation:n+B-10→Li-7+He-4 (945 barns, Q=4.79 MeV)  Equation 1

Here, the cross section and the Q value, which is the energy released bythe reaction, are shown in parenthesis.

Similarly, the interaction of a neutron with a Li-6 nucleus ischaracterized by the following equation:n+Li-6→H-3+He-4 (3840 barn, Q=2.79 MeV)  Equation 2

It is known that charged particles and heavy ions have a short range incondensed matter, generally travelling only a few microns from the pointof interaction. Therefore, there is a high rate of energy depositionaround the point of interaction. In the present invention, moleculescontaining nuclei with a high neutron cross section are mixed withmolecules that provide a scintillation response when excited by thedeposition of energy. Thus, neutron interaction with Li-6 or B-10, forexample, results in the emission of a flash of light when intermixedwith a scintillation material. If this light is transported via a mediumto a photodetector, it is then possible to convert the optical signal toan electronic signal, where that electronic signal is representative ofthe amount of energy deposited during the neutron interaction.

Further, materials such as Cd, Gd and other materials having a highthermal capture cross section with no emission of heavy particlesproduce low energy internal conversion electrons, Auger electrons,X-rays, and gamma rays ranging in energy from a few keV to several MeVemitted at substantially the same time. Therefore, a layer of thesematerials, either when mixed in a scintillator base or when manufacturedin a scintillator, such as Gadolinium Oxysulfide (GOS) or CadmiumTungstate (CWO) will produce light (probably less than heavierparticles). GOS typically comes with two activators, resulting in slow(on the order of 1 ms) and fast (on the order of 5 μs) decays. CWO has arelatively fast decay constant. Depending on the overall energy, asignificant portion of the energy will be deposited in the layer, whilesome of the electrons will deposit the energy in the surroundingscintillator. In addition, the copious X-rays and gamma rays producedfollowing thermal capture will interact in the surrounding scintillator.Thus, neutron interactions will result in events with both slow and fastdecay constants. In many cases, neutron signals will consist of a signalwith both slow and fast components (referred to as “coincidence”) due toelectron interlacing in the layer and gamma rays interacting in thesurrounding scintillator.

The scintillation response of the material that surrounds the Li-6 orB-10 nuclei can be tuned such that this light can be transported througha second scintillator, such as a plastic scintillator in one embodiment,with a characteristic which is selected to respond to gamma radiationonly. In another embodiment, the material that surrounds the Li-6 orB-10 is not a scintillator, but a transparent non-scintillating plasticresulting in a detector that is only sensitive to neutrons.

Thus, the plastic scintillator is both neutron and gamma sensitive. Whena neutron is thermalized and subsequently captured by the H in thedetector, a 2.22 MeV gamma ray is also emitted and often detected. Inthis manner, the invention disclosed in U.S. patent application Ser. No.12/976,861 achieves a composite gamma-neutron detector capable ofdetecting neutrons as well as gamma radiation with high sensitivity.Further, the composite detector also provides an excellent separation ofthe gamma and neutron signatures. It should be noted herein that inaddition to charged particles, B-10 produces gamma rays. Therefore, inusing materials that produce gamma rays following neutron capture, theresult may be a detection that looks like gamma rays. Most applications,however, want to detect neutrons; thus, the disclosed detector isadvantageous in that it also detects the neutrons.

FIG. 8 shows another embodiment of the X-ray scanning system 800 of thepresent invention that additionally uses a multi-element scattercollimator 816 to allow use of fan-beam X-ray irradiation to generatethe backscatter image. Here, the X-ray source 805 emits a fan beam 806of radiation towards the object 810. A segmented detector array 815 islocated behind a multi-element collimator 816, one detector element percollimator section. The collimator 816 is designed to permit X-rays toenter from a narrow angular range, typically less than +/−2 degrees tothe perpendicular to the detector array 815. X-rays 807 scattering fromvarious points in the object 810 which lie within the acceptance angleof, for example, the collimator element 816 are detected and associatedto the appropriate corresponding part of the generated radiographicX-ray image. Again, a sensor 820 is provided to measure distance to thesurface of the object 810 in order to correct the X-ray backscattersignal and produce a quantitative image scaled by effective atomicnumber. U.S. patent application Ser. No. 12/993,831, also by Applicantof the present invention, entitled “High-Energy X-Ray Inspection SystemUsing A Fan-Shaped Beam and Collimated Backscatter Detectors”, and filedon Nov. 19, 2010, discloses use of such a multi-element scattercollimator and is hereby incorporated by reference in its entirety.

A system configuration according to an embodiment of the inventiondisclosed in U.S. patent application Ser. No. 12/993,831 is outlined inFIGS. 9 to 11. Here, an X-ray linear accelerator 20 is used to fire acollimated fan-beam of high energy (at least 900 keV) X-radiationthrough an object 22 under inspection and to a set of X-ray detectors 24which can be used to form a high resolution transmission X-ray imagingof the item under inspection. The X-ray linear accelerator beam ispulsed, so that as the object under inspection moves through the beam,the set of one-dimensional projections can be acquired and subsequentlystacked together to form a two-dimensional image.

In this embodiment, an X-ray backscatter detector 26 is placed close tothe edge of the inspection region on the same side as the X-ray linearaccelerator 20 but offset to one side of the X-ray beam so that it doesnot attenuate the transmission X-ray beam itself. As shown in FIG. 10,it is advantageous to use two backscatter imaging detectors 26, one oneither side of the primary beam. In some embodiments the backscatterdetectors may be arranged differently. In some embodiments there may beonly one backscatter detector. In other embodiments there may be morethan two such detectors.

In contrast to known backscatter imaging detectors which use thelocalisation of the incident X-ray beam to define the scattering region,the backscatter imaging detector described, is able to spatiallycorrelate the intensity of backscattered X-ray signals with their pointof origin regardless of the extended fan-beam shape of the X-ray beam.

In the backscatter imaging detector 26, this spatial mapping isperformed using a segmented collimator 28 in zone plate configuration asshown schematically in FIG. 11. Normally, a zone plate will comprise aseries of sharply defined patterns whose impulse response function iswell known in the plane of a two-dimensional imaging sensor that islocated behind the sensor. In the present case, the energy of the X-raybeam to be detected is typically in the range 10 keV to 250 keV and sothe edges of the zone plate pattern will not be sharp. For example, azone plate fabricated using lead will require material of thicknesstypically 2 mm to 5 mm. Further, it is expensive to fabricate a highresolution two-dimensional imaging sensor of the size that is requiredin this application.

However, it is noted that the radiation beam is well collimated in onedirection (the width of the radiation fan beam) and therefore theimaging problem is reduced to a one-dimensional rather than atwo-dimensional problem. Therefore a backscatter detector in the form ofan effectively one dimensional imaging sensor 30 is provided behind thezone plate 28. To address this problem an elemental backscatter detectoris used in this embodiment. As shown in FIG. 11, the detector 30comprises a plurality of detector elements 32. FIG. 12 illustrates adetector element 32 suitable for use in this example. Here, the detectorelement 32 comprises a bar of scintillation material (about 100 mm longin this example) and is supplied with a photo-detector 34 at either end.The photo-detector 34 may advantageously be a semiconductor photodiodeor a photomultiplier tube. X-ray photons that interact in thescintillation material emit light photons and these will travel to thetwo photo-detectors where they may be detected. It may be shown that theintensity of the light reaching each photo-detector is in proportion tothe distance of the point of interaction from the face of thephoto-detector. Therefore, by measuring the relative intensity at thetwo photo detectors, the point of interaction of the X-ray photon withthe detector can be resolved.

Referring back to FIG. 1A, the covert surveillance vehicle 105 isequipped with a plurality of other sensors 110, apart from the X-rayscanning system, in accordance with an aspect of the present invention.In one embodiment, the vehicle 105 is equipped with a GPS receiver theoutput of which is integrated with the on-board X-ray scanning system toprovide the absolute location at which each scan line is conducted.Again, output from a scanning laser is reconstructed into a 2D image toprovide a quantitative analysis of the scene around the vehicle. This 2Dimage is archived for subsequent analysis and review.

The 2D laser scanner image may also be used to determine when theoverall scan of a particular object should start and when the scan forthat object is complete.

Also, optical wavelength colour CCTV images are collected at the frontand sides of the vehicle, ideally using pan-tilt-zoom capability, toallow clear review of all locations around the vehicle. In oneembodiment, images from the CCTV cameras are analysed to read licenseplate and container codes and this data is also archived along with theX-ray, GPS and all other surveillance data. Similarly, infra-red camerascan also be used to monitor the scene around the vehicle to look forunexpectedly warm or cold personnel as indication of stress or presenceof improvised explosive devices. This data is also archived along withX-ray and all other surveillance data.

In one embodiment, audio microphones are also installed around thevehicle to listen for sounds that are being produced in the vicinity ofthe vehicle. Specialist microphones with pan-tilt capability areinstalled to listen to sounds from specific points at some distance fromthe vehicle, this direction being analysed from the CCTV and IR imagedata.

Directional RF (Radio Frequency) antennas are installed in the skin ofthe vehicle to listen for the presence of electronic devices in thevicinity of the vehicle. This data is integrated with the rest of thesurveillance data. Similarly, wide band antennas are installed withreceiving devices that monitor communications channels that may be usedby law enforcement, military and emergency services. Again, RF antennasare installed to monitor mobile phone communications including textmessaging from the local region around the vehicle.

In one embodiment, chemical sensors are also installed to monitorcomposition of the air around the vehicle to detect trace quantities ofexplosives, narcotics and other relevant compounds with this data beingintegrated with that generated by the imaging and other sensors.

In accordance with another aspect of the present invention, an automateddetection processor integrates and analyses all surveillance informationfrom the plurality of sensors 110, in real-time, to highlight threatitems for review by an operator seated inside the vehicle 105 and/orremotely through a secured wireless network. In one embodiment, datafrom the individual sensors is analysed for key signatures. For example,the X-ray data is analysed for detection of improvised explosive devicesor for the presence of organic materials in unexpected places (such asthe tyres of a car). CCTV data is analysed for license plates withcross-checking against a law enforcement database. Audio information isanalysed for key words such as “bomb” or “drugs”, for unexpectedly fastor deliberate phrasing which may indicate stress, or for a non-nativelanguage in the presence of a native language background for example.Once a piece of information has been analysed to comprise a threat orrisk, this is escalated up a decision tree and is then compared againstautomated risk analysis from other sensors. If correlated risks aredetected, a significant threat alarm is raised for immediate action by ahuman operator. If no correlated risk is detected, a moderate threatalarm is raised for review by the operator. The result is a managed flowof information where all sensor surveillance information is analysed atall times, and only significant threat information is passed up thedecision tree to reach the final level of an alert to a system operator.The detection processor, in one embodiment, is a microprocessor computerrunning relevant code programmed for managing information and decisionflow based on correlation and aggregation of the plurality ofsurveillance information.

Great Britain Provisional Patent Application Number 1001736.6, entitled“Image Driven Optimization”, and filed on Feb. 3, 2010, and PatentCooperation Treaty (PCT) Application Number GB2011/050182 entitled“Scanning Systems”, and filed on Feb. 3, 2011 by the Applicant of thepresent specification, both herein incorporated by reference in theirentirety disclose a scanner system comprising a radiation generatorarranged to generate radiation to irradiate an object, and detectionmeans arranged to detect the radiation after it has interacted with theobject and generate a sequence of detector data sets. Referring to FIG.13, a scanner system comprises an X-ray beam generation system whichincludes a shielded radiation source 10, a primary collimator set 12Aand a secondary collimator set 12B, and a set of radiation detectors 14configured into a folded L-shaped array 16, are disclosed.

The primary collimator set 12 A acts to constrain the radiation emittedby the source 10 into a substantially fan-shaped beam 18. The beam 18will typically have a fan angle in the range +/−20 degrees to +/−45degrees with a width at the detector elements 14 in the range 0.5 mm to50 mm. The second collimator set 12B is adjustably mounted and theposition of the two second collimators 12B can be adjusted by means ofactuators 20, under the control of a decision processor 22. Thedetectors 14 output detector signals indicative of the radiationintensity they detect and these form, after conversion and processingdescribed in more detail below, basic image data that is input to thedecision processor 22. The decision processor 22 is arranged to analysethe image data and to control the actuators 20 to control the positionof the second collimator set 12B in response to the results of thatanalysis. The decision processor 22 is also connected to a control inputof the radiation source 10 and arranged to generate and vary a controlsignal it provides to the control input to control the energy and timingof X-ray pulses generated by the radiation source 10. The decisionprocessor 22 is also connected to a display 24 on which an image of theimaged object, generated from the image data, can be displayed.

By way of example, the radiation source 10 may comprise a high energylinear accelerator with a suitable target material (such as tungsten)which produces a broad X-ray spectrum with a typical beam quality in therange from 0.8 MV to 15 MV from a relatively small focal spot typicallyin the range 1 mm to 10 mm diameter. The radiation source 10 in thiscase would be pulsed with a pulse repetition frequency generally in therange 5 Hz to 1 kHz where the actual rate of pulsing is determined bythe decision processor 22.

The detectors 14 in this case are advantageously fabricated from a setof scintillation crystals (generally high density scintillator such asCs1, CdW04, ZnW04, LSO, GSO and similar are preferred) which areoptically coupled to a suitable light detector, such as a photodiode orphotomultiplier tube. Signals from these detectors 14 converted todigital values by a suitable electronic circuit (such as a currentintegrator or trans impedance amplifier with bandwidth filteringfollowed by an analogue to digital converter) and these digital valuesof the sampled intensity measurements are transferred to the decisionprocessor 22 for analysis. The primary 12 A and secondary 12Bcollimators in this case are advantageously fabricated from high densitymaterials such as lead and tungsten.

A plurality of active devices are installed on the vehicle 105 to helpmitigate against threats that may be present proximate to the covertinspection vehicle itself. For example, a jamming device can beinstalled to block mobile phone communication. This device may be turnedon automatically in certain situations based on results from theautomated decision processor. For example, should an improvisedexplosive device be detected in the vicinity of the vehicle the jammingdevice is turned on automatically to block spoken commands to asubversive or to prevent direct communication to the trigger of theexplosive device. A jamming device can also be installed to blocksatellite communications required in order to prevent satellite phonecommunications that may result in subversive activity.

In one embodiment the covert inspection vehicle 105 is operated by asingle person with the primary responsibility for driving the vehicle.Surveillance data can be broadcast back to a central intelligencelocation in real time, as required, with download of the full archivedsurveillance data once the vehicle returns to its home location. Theautomated decision processor can action or trigger appropriate events,depending upon the decision steps programmed therein, without operatorintervention to avoid the driver loosing focus on their primary task. Inanother embodiment, the covert inspection vehicle 105 is also providedwith space for another security operative whose task is to monitor thesurveillance data stream as it arrives from the plurality of sensorseither in parallel with the automated decision processor or as aconsequence of information from the automated decision processor. Thisoperator is provided with two way secure wireless communication back toa central intelligence location in order to transact instructions andactions as required.

The above examples are merely illustrative of the many applications ofthe system of present invention. Although only a few embodiments of thepresent invention have been described herein, it should be understoodthat the present invention might be embodied in many other specificforms without departing from the spirit or scope of the invention.Therefore, the present examples and embodiments are to be considered asillustrative and not restrictive, and the invention may be modifiedwithin the scope of the appended claims.

I claim:
 1. A method for obtaining an atomic number (Z) of each material contained in an object being scanned by a covert mobile inspection vehicle comprising: a backscatter X-ray scanning system comprising an X-ray source and a plurality of detectors for obtaining a radiographic image of the object; at least one sensor for determining a distance from at least one of the plurality of detectors to points on a surface of the object; and a processor for processing the obtained radiographic image by using the determined distance of the object to obtain an atomic number of each material contained in the object; the method comprising the steps of: determining a boundary of each region of the radiographic image by using a statistical filter; calculating a standard deviation of energies of pixels present in each region; calculating a product of the obtained standard deviation and a mean of the energies of pixels present in each region; and comparing the calculated product to a pre-determined scale where a low value of the product corresponds to a low Z material and a high value of the product corresponds to a high Z material.
 2. The method of claim 1 wherein the at least one sensor is a scanning laser range finder causing a beam of infra-red light to be scattered from the surface of the object and wherein a time taken for the beam of infra-red light to return to the at least one sensor is indicative of the distance from at least one of the plurality of detectors to the surface of the object.
 3. The method of claim 1 wherein the processor is configured to cause an intensity correction to be applied to the obtained radiographic image thereby causing an intensity of an image of an object located at a distance greater than a predefined distance to be reduced by a predefined factor and an intensity of an image of an object located at a distance lesser than a predefined distance to be increased by a predefined factor.
 4. The method of claim 1 wherein the plurality of detectors comprise at least one of an inorganic scintillation detector, a detector comprising NaI(Tl), an organic scintillator, and a detector comprising polyvinyl toluene and wherein each of said plurality of detectors is coupled with at least one light sensitive readout devices.
 5. The method of claim 1 wherein the plurality of detectors comprise at least one of CdTe, CdZnTe and HgI.
 6. The method of claim 1 wherein the plurality of detectors comprise HPGe.
 7. The method of claim 1 wherein the covert mobile inspection vehicle further comprises at least one of: a GPS receiver, a scanning laser, a CCTV camera, an infra-red camera, an audio microphone, a directional RF antenna, a wide-band antenna, a chemical sensor, and a jamming device.
 8. The method of claim 1 wherein, using said statistical filter, determining if a given pixel is part of a main scattering object, or part of an adjacent object.
 9. The method of claim 8 wherein, if a given pixel is part of an adjacent object, not using a value of said pixel in a calculation of a standard deviation of energies for a region that includes the main scattering object.
 10. The method of claim 1 wherein the statistical filter generates a list of pixel values for a region, ranks the pixel values in order, determines a mean of energies of pixels present in the region, and determines a standard deviation of energies of pixels present in each region.
 11. The method of claim 1 wherein the X-ray source has an X-ray tube with a cathode-anode potential difference in a range 160 kV to 320 kV.
 12. The method of claim 11 wherein the X-ray source has a tube current in a range of 1 mA to 50 mA.
 13. The method of claim 1 further comprising using the calculated product to discriminate between gamma rays and wherein the X-ray source has a tube current in a range of 1 mA to 50 mA.
 14. The method of claim 1 wherein the X-ray source comprises a linear accelerator with a target material configured to produce an X-ray spectrum with a beam quality in a range from 0.8 MV to 15 MV.
 15. The method of claim 14 wherein a focal spot of the X-ray source has a diameter in a range of 1 mm to 10 mm.
 16. The method of claim 1 further comprising pulsing the X-ray source with a pulse repetition frequency in a range of 5 Hz to 1 kHz. 